1. Field of the Invention
The present invention relates to a light emitting element. In particular, the invention relates to an organic electroluminescent element (organic El element) capable of obtaining high-efficiency, longer-wavelength emission in a red light emitting element.
2. Description of the Related Art
When an electric field is applied to both electrodes of an organic EL element, electrons are injected thereto form a cathode and holes are injected thereto form an anode. These electrons and holes recombine in a luminescent layer to release energy as light when the electron undergo the transition from the conduction band to the valence band, which causes the organic EL element to emit light.
When an organic EL element is utilized as a full color display, the EL element needs to have red, green and blue (RGB) light emitting materials that offer appropriate chromaticity and sufficient luminance efficiency. For the achievement of this object, a system in which a variety of light emitting materials providing a desired color are doped in charge transport materials having optimum transport and luminescence properties has been used.
Preferable red light emitting materials for a luminescent layer used in the above-described system include, for example, a 4H-pyranylidenepropanedinitrile derivative disclosed in Japanese Patent Application Laid-Open (JP-A) No. 10-308281. This compound exhibits an emission peak at a relatively long wavelength and provides red emission of high luminance efficiency. Even using these compounds, however, does not provide a sufficient peak value of a luminescence wavelength from the standpoint of color, and thus a material is presently required that exhibits high luminance efficiency and emits light at a longer wavelength.
Examples of a luminescent layer of a light emitting element, in which the layer adopts a laminated structure with repeating units, include those of JP-A No. 2003-187977 and International Publication WO01/08230A1. JP-A No. 2003-187977 describes an organic EL element wherein the element has three or more luminescent layers alternately laminated, and the luminescent layers emit beams of different peak wavelengths. More specifically, this document relates to a white light emitting element having both yellow light emitting material layers of rubrene and blue light emitting material layers of perylene alternately laminated three or more times each. Also, International Publication WO01/08230A1 discloses, as a luminescent layer having a laminated structure having repeating units, a luminescent layer that has both a luminescent layer comprising a light emitting material (DCM2) radiating red fluorescence and a luminescent layer comprising a triplet radiative indium complex alternately laminated.